Tubular equipment, tubes and pipes, which are taken to be synonymous in this specification, are used in industrial, commercial, residential, and municipal applications for many different purposes ranging from transport of drinking water to processing mixtures of toxic chemicals.
One such use of tubular equipment is for the transport of fluids, such as gases, liquids, mixtures of gases and liquids, and slurries comprising mixtures of liquids and solids. In many cases, deposits of solid material accumulate on the walls of the tube and restrict fluid flow therethrough. This increases the power required to transport the fluid, see for example, Kipin, Peter, "Cleaning Pipelines: a Pigging Primer", Chem. Eng., Feb. 4, 1985, pp 53-58. Examples of this type of behavior are the build-up of paraffin waxes on the inside of pipes used to transport petroleum products described in Pipeline Pigging Technology, Tiratsoo, J. N. H., editor, Gulf, Houston, 1988 and the build-up of iron oxide corrosion products on the inside of steel water pipes described in aforesaid Kipin, Peter, "Cleaning Pipelines: a Pigging Primer".
Another common use for tubes is as heat exchange equipment in the indirect heating or cooling of fluids and fluid-solid mixtures through the wall of the tube. It is common during these processes for an insoluble scale to form on the inside of the tube due to a change in solubility of solute in the fluid due to a change in temperature by heating or cooling, as described in Someah, Kaveh, "On-Line Tube Cleaning: the Basics", Chem. Eng. Prog. July 1992, pp 39-45. A common example of this is the heating of water in water-cooled steam-condensers in power plants. As the cooling water in the tube is heated by condensing steam the solubility of certain inorganic salts such as calcium carbonate and calcium sulphate is decreased and these salts precipitate out of the water and accumulate on the tube wall. Heat sensitive materials when heated through the tube wall sometimes will foul the inside of tubes when the material is chemically altered by overheating. The caramelization of sugar solutions during heating is an example of this common problem.
In addition to causing an increase in flow resistance and, thus, power required to maintain the flow, the build-up of scale and other forms of fouling greatly impairs the transfer of heat to the fluid through the fluid wall. Said impairment of heat transfer necessitates that larger and more expensive equipment be employed, compared to operation without scale or fouling.
Tubes are also used to provide residence time for reacting flows of fluids and fluid-solid mixtures. In such reacting systems it is often the case that solids will build-up on the wall of the pipe due to settling of solids from the suspension, precipitation of a reaction product, or a reduction in solubility of a non-reacting component in the stream due to a change in fluid temperature in the reactor. An example of a reacting system which exhibits the latter phenomena is supercritical water oxidation of organic wastes containing inorganic material as described in U.S. Pat. No. 5,252,224--Modell et al, issued Oct. 12, 1993. In the process described in U.S. Pat. No. 5,252,224, the reaction is carried out under adiabatic conditions. As the reaction progresses, the heat of reaction raises the fluid temperature and inorganic salts tend to precipitate out of the fluid phase and have been known to cause build-up of solids in tubular reactors. Such build-up of solids leads to an increase in flow resistance and, thus, an increase in the consumption of energy for the transport of fluid through the reactor. If these solids are allowed to continue to accumulate, complete plugging often occurs and results in the costly event of lost production from the facility.
The use of cleaning members or pigs for the purpose of cleaning tubular equipment, such as pipelines and municipal and industrial process piping and equipment is well known. Cleaning members are used to remove both liquid and solid material from the lines and are usually propelled by a fluid through the hydraulic forces exerted on the cleaning member by the flowing fluid.
In the cleaning of petroleum pipelines a cleaning member known as a pig is introduced in the pipeline at a pig launching station and moved through the pipeline by the hydraulic force exerted by a motive fluid, usually the petroleum product transported through the line. Some distance downstream, the cleaning member is received into a pig receiving or catching station. A pig can be loaded into a pig launcher, manually or automatically and similarly unloaded from the pig receiver manually or automatically. However, the pig must be transported back upstream to the launching station by independent means, usually by surface transport.
In the chemical, food, and related process industries pigs are known to be used to clean process tubes and to act as a barrier between fluids so that a single tube can be used to transport several different fluids or fluid-solid mixtures without intermixing of the fluids. In such systems pigs are often introduced and withdrawn from the tubes using pig launchers and catchers in the same manner as described above for petroleum pipelines. In alternative systems, pigs are launched and received by the same device by reversal of the direction of flow of the motive fluid in the tube as described in U.S. Pat. No. 3,883,431--Ishii et at, issued May 13, 1975, U.S. Pat. No. 4,198,293--Ogawa et al, issued Apr. 15, 1980, U.S. Pat. No. 4,607,410--Bersch, and U.S. Pat. No. 5,072,476--Bersch, issued Dec. 17, 1991. It is known that such systems can be automated to periodically reverse the flow of the fluid through the tube, and by using a launching and receiving device at each end of the tube the cleaning member can periodically be pushed through the tube by the fluid in alternating directions. Unfortunately, for many of the flow systems which use tubes and require periodic cleaning, the periodic reversal of flow through the tube is not tolerable and thus this method cannot be used.
Methods are known using the continuous circulation of rubber cleaning balls through process tubes, for example, U.S. Pat. No. 4,620,589--Koller, issued Nov. 4, 1986, U.S. Pat. No. 4,566,533--Bochinski et al, issued Jan. 28, 1986, U.S. Pat. No. 5,086,833 Ben-Dosa, issued Feb. 11, 1992. In these methods, such balls are loaded into the entrance of the process tube by an automatic loading device and pushed through the tube by the hydraulic force of the process fluid on the balls out through an exit. After the exit of the tube the balls are separated out of the process fluid by a screening device and automatically removed from the process to a recirculation system which returns the balls to the automatic loading device. Unfortunately, the use of such cleaning systems is limited to process conditions which are compatible with the material of construction, such as rubber, of the balls, and as such cannot be used to clean systems operating at high temperature or under other conditions which are detrimental to the rubber. In addition, the methods and apparatus used for loading, removing, and recirculating the cleaning members are only useful when the cleaning member is essentially spherical.
U.S. Pat. No. 3,425,083--Wennerberg et at, issued Feb. 4, 1969 describes a device for cleaning an endless tube. The operation of this device requires that the flow of fluid through the tube is greater than the flow entering and exiting the system, such that a portion of the fluid that has passed through the tube is recirculated from the "end" of the loop to the "beginning" of the loop allowing recirculation of the cleaning member. Eduction of this recirculating fluid is effected with a second large flow recirculation loop and pump. In addition to requiring substantial energy to effect the eduction of the recirculating liquid, this device is deficient in that a large portion of the fluid exiting the tube must be mixed with the feed to the system.
In addition to the pigging methods described above, which can be employed in some circumstances to effect tube cleaning without the interruption of fluid flow, methods exist to clean tubes which require the tubes to be taken out of useful service. These methods include mechanical means such as bristle brushes, rubber plugs, scrapers, cutters, vibrators, and water lances. These methods have the undesirable feature that they require the process tube and attached equipment to be taken out of service during the cleaning operation, and in addition often require that the equipment be partially disassembled prior to cleaning. An additional method which also requires the tube to be taken out of service is cleaning by chemical means. For example, many types of scale which form on heat exchanger tubes can be removed by washing the tube with nitric acid. Chemical cleaning methods have the further disadvantage of requiring disposal, or clean-up and recycle, of the cleaning chemicals.
It is therefore clear that there exists a need for a system which will allow the cleaning of a process tube by tube cleaning members on a periodic basis, without disrupting the continuous unidirectional flow of the process fluid through the tube, without being limited to the use of spherical rubber cleaning members, without requiring mixing of the inlet and outlet streams, and without requiring an additional pump to provide the means of recirculating the cleaning members.